Sientra 285cc vs 315cc, is there much of a difference?

Thank you for the question. Depending on the patient's body type and goals, a 30 cc difference in breast implant volume may or may not create a "much of a difference".  For example,  for a more petite patient, even small increments in volume may make a significant difference in final outcome;  for "larger" patients, the same difference in volume may not be clinically noticeable.
Ultimately, careful selection of plastic surgeon and careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.
Given that your surgery is coming up soon, I would suggest that you spend additional time communicating your goals/concerns directly with your plastic surgeon, preferably prior to the date of surgery. One question that might be helpful to you: will you be more disappointed if you are slightly too small or slightly too big?
 Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. ***Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, I have found that the use of words such as “natural” or "C or D cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this (and the attached link, dedicated to breast augmentation surgery concerns) helps. Best wishes for an outcome that you will be very pleased with. 

Thank you for your question. "Tissue based planning" relies on your chest and breast measurements and is the safest way to ensure good long term results. Your surgeon should determine what size and projection are necessary to give you your desired result based on your body's measurements. The best way to assess and give true advice would be an in-person exam. Please see a board-certified plastic surgeon that specializes in aesthetic breast surgery. Hope this helps and good luck with your surgery.

The 30 cc difference is equivalent to an ounce or a shot glass in volume.  Visually, you will look a little fuller.  You should be able to go with the larger implant.  0.1 mm in width is not significant between the implants.